Forests cover around 30 % of the Earth’s surface, are home to millions of species, capture and store carbon dioxide (CO2), influence the local climate and provide us with water. This month, Horizon examines how the health of the world’s forests impacts on climate change. We explore the link between deforestation, forest degradation and CO2 emissions, examine the push to combine farming and forestry for better land management, and find out how scientists are using tree rings and DNA to combat illegal logging.